[Boyka96]

thats a good question .. the difference is the speed .. mysql_pquery is faster, however it depends on how many connections you have (how many databases are connected at the same time) .. anyway both of them have the same effects: execute a query and triggers a callback may also send some values to the callback (playerid or whatever).
P.S: I'm not sure if thats the only difference but atleast I'm sure of that.